1. Field of the Invention
The present invention relates in general to a method for the production of polyester for a film with excellent surface properties, and more particularly to a method for the production of polyester including silica particles therein which are capable of allowing a film molded of the polyester to have excellent surface properties such as slip property and abrasion resistance. The present invention also relates to a method for the production of dispersion-in-ethylene glycol type silica sol capable of being dispersed evenly well in a polyester film.
2. Description of the Prior Art
As well known to those skilled in the art, polyester, of which polyethylene terephthalate is representative, has excellent intrinsic physical and chemical properties, and is capable of being formed into a film, which has been recently used for various purposes, for example, for a magnetic tape, a graphic art film, a condenser film, a package film and the like.
However, in spite of its excellent intrinsic properties, the film which is molded of polyethylene terephthalate, a representative of polyester, has a large coefficient of friction and is liable to generate static electricity, so that there may occur many problems in a series of the processes for molding polyethylene terephthalate into a film, for example, a magnetic tape, such as a process of forming a film therewith, a process of coating the film with a magnetic material and the like.
For example, in a process for the production of a film, friction occur between the film and the roll through which the film is passed in the process. In accordance with the occurrence of the friction, there also occurs abrasion on the film. Meanwhile, the friction and the abrasion are dependent on the slip property and the abrasion resistance of the film, respectively. That is, the slip property and the abrasion resistance of the film are the properties that resist the friction and the abrasion. Therefore, the film having the poor slip property and abrasion resistance does not resist the friction and abrasion, effectively, so that scratches are generated on the film. In addition, the friction and the abrasion allow a quantity of white powder to occur on the film surface. As a result, the poor slip property and abrasion resistance of a film detrimentally affects the quality of final products made of the film. Accordingly, the slip property and abrasion resistance of a film is one of the most important factors that determine the quality of the film and the film-made product.
Particularly, in the event of using a polyester film as a material for a magnetic tape, which is formed, for example, by applying a magnetic layer to the polyester film surface, when the magnetic tape is running in a recording/playback apparatus in order to record magnetic signals thereon or to play back the magnetic signals recorded thereon, the tape comes into long contact with a head drum and a guide roll in the apparatus. Therefore, scratches and white powders are more likely to occur on the surface of the magnetic tape due to the long contact. Subsequently, the scratches and the white powders detrimentally affect the magnetic tape such as to generate the drop out of magnetic signals. In addition, if the magnetic tape is used repeatedly many times and for a long time, coefficient of friction is increased, so that the traveling ability of the magnetic tape becomes deteriorated.
In an effort for solving the above-mentioned problems, there is generally used a method for the production of polyester which seeks to reduce the contact areas between the films themselves made of polyester and between the film and the roll by applying on the surface of the film protrusions, which improve the slip property and the abrasion resistance of the film.
Prior arts have sought ways of applying the protrusions on a film surface and thus, improving the film surface properties in a satisfactory manner.
For example, a method of forming inner particles is disclosed in Japanese Patent Publication No. Sho. 52-32914 wherein inactivated fine particles are deposited from catalyst residues during the production of polyester.
Another example is a method of casting outer particles that is disclosed in Japanese Patent Publication No. Sho. 56-19263 wherein inactivated fine particles are added during the production of polyester.
In the method of forming inner particles, there are such advantages that it is not necessary to pulverize and grade the particles, the deposited particles have a good affinity for the film, and the particles has excellent abrasion resistance due to their low hardness. This method, however, has difficulty in controlling the diameter of the deposited particles, and there occur differences among the batches which are made by the method.
The method of casting film with outer particles, in which inactivated inorganic particles such as calcium carbonate, barium sulfate, calcium sulfate, kaolin, silica, talc, titanium dioxide and the like are added with polyester is synthesized or polyester is molded into a film, is advantageous in the fact that it is possible to freely select the amount and the diameter of particles, and to stabilize the conditions of particles and the properties of polymer even when operating for long time. However, in dependence on the kind and the size of the particles, macro granules are formed by aggregation of the particles, so that there occur disadvantages such as, for example, drop out and the like when the film is used, for example, as a magnetic tape.
In general, the more particles are included in polyester, the better is the slip property of the film made of the polyester. However, the particles with large diameters which are included in polyester film in order to improve the slip property thereof may lower the quality of the final product made of the polyester film, such as a video tape, an audio tape and the like, causing the product used in such a precision field as an electromagnetic field to be deteriorated in electromagnetic transit property.
Recently, an electromagnetic recording medium has been required to have a larger capacity in a smaller size or to be compact. Thus, according to the requirement, the components for recording electromagnetic signals are necessary to be highly dense in a film for the electromagnetic recording medium. Subsequently, this causes the particles which are added to or deposited from the film to become finer.
However, more fine particles have a strong tendency to aggregate to one another because attraction among the fine particles becomes stronger, so that the number of macro granules which result from the aggregation of the fine particles becomes increased on a film. Thus, it is demanded to disperse the fine particles evenly on the film to prevent the particles from the aggregating one another.
For example, for the purpose of improving the slip property of the film molded of polyester, silica which is an inorganic particle, is added during synthesizing polyester. However, the silica has a strong tendency to aggregate in the polyester film. As a result, there are formed macro granules, and their number is increased more and more in the polyester film.
In this case, in order to prevent macro granules from occurring, such various methods as can disperse the silica particles evenly are disclosed. For example, in Japanese Patent Publication No. Sho. 57-44623, disclosed is a method in which silanol groups of the silica particle are blocked to improve the dispersibility of the silica particles. Another method is disclosed in Japanese Patent Laid-Open No. Sho. 63-221158 in which monodispersive silica particle spheres that result from the hydrolysis of an organometallic compound, for example, ethylorthosilicate, are added during the synthesis of polyester.
The above-mentioned methods are capable of obtaining protrusions on the film which are uniform to some degree. It, however, is known that during the polymerization of polyester, there occur hydroxy groups on the surfaces of silica particles, which are one of the factors that attract the particles one another, so that when the polyester is molded into a film, macro granules are formed on the surfaces of the polyester film.